Revenues from Contracts with Customers - Schedule of Disaggregated Reported Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 8,601	$ 25,159
Total service revenue
Disaggregation of Revenue [Line Items]
Total revenue	5,699	17,201
Postpaid
Disaggregation of Revenue [Line Items]
Total revenue	4,236	12,679
Prepaid
Disaggregation of Revenue [Line Items]
Total revenue	924	2,860
Wholesale, affiliate and other
Disaggregation of Revenue [Line Items]
Total revenue	294	881
Wireline
Disaggregation of Revenue [Line Items]
Total revenue	245	781
Equipment sales
Disaggregation of Revenue [Line Items]
Total revenue	1,589	4,180
Equipment rentals
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,313	$ 3,778